Semiconductor elements operating in the high-frequency region are conventionally used in electronic devices including mobile phones and various communication equipments and various dielectric ceramics have been proposed as the substrate materials for mounting such semiconductor elements.
Of those dielectric ceramics, silicon carbide is particularly known to have high mechanical strength and stable chemical properties in combination. However, because of the large loss in the high-frequency region, silicon carbide that was used in conventional mechanical parts and the like was not suitable for the material of a substrate on which a high-frequency semiconductor element is to be mounted.
Then, for instance, Patent Literature 1 discloses a silicon carbide substrate composed of polycrystalline silicon carbide and having reduced loss in the high-frequency region by, for example, applying heat treatment.